GTX750ti or 1050 or 1050ti, it depends, which one you can find easier and cheaper.
I would also try to get a second-handed Q6600 or better, as far as your BIOS allows.
Having at leat 8GB RAM is also very helpful.
...and...try to plan a big upgrade and move on to a newer platform e.g. Ryzen, Coffeelake, etc.